We already have Ghostscript which can rasterize and has been embedded in "Postscript compatible" laser printers.

I would imagine that printers ride this line where there's not enough benefit to reverse engineer all the fiddly mechanical bits to make it print, extremely limited hardware expansion, and unlike router firmware or general purpose compute software there's little additional new functionality that would be possible.

That said, there are people who hacked printers to make them do other things - for example, the ThunderScan which let certain old dot matrix Apple printers scan: https://www.youtube.com/watch?v=yoUYqLteH9Q but that still needed additional hardware added.